It is known to control the instant of ignition, in internal combustion engines with external supplied ignition, in open- and closed-loop fashion in accordance with various operational parameters of the engine. In addition, the instant of ignition has also been varied in accordance with the composition of the exhaust gas, see German Offenlegungsschrift (laid-open application) 27 400 44.
It is also known to control the instant of ignition in a closed-loop fashion in accordance with the intensity of knocking combustion in such a way that an amount of still-acceptable hard combustion is retained, see German Offenlegungsschrift (laid-open application) 28 01 512.
Furthermore, it is known to utilize ionic current sensors for detecting the combustion process in the combustion chambers of internal combustion engines. As disclosed in German Offenlegungsschrift (laid-open application) 24 43 413, the spreading speed of the flame front is measured in the combustion chamber of the engine during sequential work cycles with the aid of an ionic current sensor after ignition of the operational mixture.
The fluctuations in these spreading speeds or running times provide an indication of the approach toward the running limit of the engine. With an increasing air number .lambda., the relative fluctuations in the running time increase.